(CNN)-- The United States faced violent protests outside embassies in Egypt and Yemen on Thursday, even as it was deploying warships as part of the hunt for those behind the deadly attack against U.S. personnel in Libya.

But sources tracking militant Islamist groups in eastern Libya say the deadly attack Tuesday night that killed four U.S. personnel, including Ambassador Chris Stevens, was most likely carried out bya pro-al Qaeda group. President Obama has vowed that "justice will be done" after the attack on the U.S. Consulate in Benghazi.

Warships, carrying guided missiles, and unmanned dronesare being sent to help search for the killers.

A group of Marines called a Fleet Antiterrorism Security Team was deployed to Libya tohelp secureU.S. facilities, two U.S. officials said. About 50 Marines were headed to Tripoli and could deploy elsewhere in Libya after their arrival, the officials said.

While Libya, Egypt and Yemen were of primary concern, there were protests at other U.S. embassies as well.

The United States called on U.S. citizens to stay away from the embassy in Khartoum, Sudan, on Wednesday, amid protests.

Andphotosshowed protesters in front of U.S. embassies in Tunisia and Morocco. Palestinians demonstrated outside U.N. headquarters in Gaza, and Palestinian men burned a U.S. flag at a demonstration in Gaza City.

One photo from Cairo depicts a Jewish star with the words in Arabic, "Remember your black day 11 September."